1. Field of the Invention
This invention relates to circuitry for the regeneration at a facsimile receiver of the carrier signal from a signal which includes an amplitude modulated suppressed carrier signal plus periodic periods of unmodulated carrier. In particular, the invention relates to such circuitry utilizing a phase lock loop to obtain the carrier signal.
2. Description of the Prior Art
There are facsimile machines which use an amplitude modulated vestigial side band (AMVSB) system wherein the signal contains a net carrier signal which is available at the receiver for recovery and use in synchronously demodulating the amplitude modulated signal. This net carrier signal does not contain useful information data. Since commercial telephone lines used for interconnecting facsimile machines limit the total power level of the signal that can be transmitted, it is desirable that the net carrier be suppressed. The resulting suppressed carrier system provides a greater signal to noise ratio than an unsuppressed carrier system. Elimination of the net carrier, however, requires that a suitable way be found to regenerate the carrier signal at the facsimile receiver for synchronously demodulating the received AMVSB suppressed carrier signal.
Standards have been adopted to ensure compatability between facsimile machines of different manufacture when operated in an amplitude modulation mode, which includes AMVSB suppressed carrier systems. The standards provide for an initial synchronizing time period (approximately 6 seconds) during which full carrier is transmitted. During this initial time period, the full carrier is interrupted regularly (once every scan line) to provide a short phase period (10 m. seconds) of no carrier. The initiation of these regular short interruptions is controlled in the case of a drum-type facsimile by the phase or "start" position of the transmitter drum. These regular short interruptions are phasing periods and are used by the receiving facsimile machine to synchronize the position of its drum or comparable device to that of the transmitter drum. Once synchronization is completed, the transmitter begins to scan the document and to transmit the scanned information via the AMVSB suppressed carrier system. In such a system, unmodulated carrier is present during the scan of white areas of the document and during each of the short phase periods which continue to occur regularly during the transmission operation.
The carrier signal needed at the receiver for demodulation of the AMVSB suppressed carrier signal could be obtained by full wave rectifying the received signal and feeding the resultant doubled frequency signal to a phase lock loop (PLL). This technique may work; however, there is no guarantee that there will always be carrier during the document transmission, e.g., no carrier exists for a black copy. Each phase period is known to contain full carrier, but it is present only for a brief time each scan line. A conventional PLL system would drift and jitter considerably when locked on such a signal. Furthermore, any carrier portions provided in the received signal that correspond to white areas in the document being scanned are not consistently of a sufficient duration to provide a basis for the regeneration of the carrier without the presence of the foregoing problem.